dt-bindings: interrupt-controller: Add T-HEAD's TH1520 PLIC

Add compatible string for T-HEAD TH1520 plic.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>